1. Field of the Invention
This invention relates to loudspeakers, and is applicable particularly, but not exclusively to loudspeakers intended to be hung adjacent a wall, in the manner of a picture.
2. Description of the Related Art
Flat loudspeakers have been known for many years, for example free-standing electrostatic loudspeakers. However, such loudspeakers have had to be large and therefore obtrusive in order to produce a satisfactory sound level. They have been expensive and have had a less than desirable sound frequency response and sound distribution pattern.
Various other types of flat, wall-mountable loudspeakers have been devised, but again they have suffered variously from inferior frequency response or stereophonic performance.
According to one aspect of the present invention there is provided a loudspeaker including a substantially planar first diaphragm which can be vibrated so as to radiate sound from at least the front face thereof, and a driver unit operable by a varying electric current in order to generate a varying force on the first diaphragm, the force varying in a manner related to the varying electric current, characterised by the driver unit being connected to the first diaphragm at one or more selected positions or mounted to structure which is in turn connected to the diaphragm at one or more selected positions.
Preferably the material of the diaphragm is anisotropic (with different characteristics in each of its major axis).
More preferably the material (without apertures) has a bending strength in one direction of the plane of the material which is significantly greater than the bending strength along a line in the plane of the material at right angles to the first direction.
In its most preferred form it has a longitudinal bending strength which is about twice its transverse bending strength, which is particularly suitable to panels which are rectangular or elliptical.
Preferably the loudspeaker includes a substantially rigid peripheral frame having a front face and a rear face, said first diaphragm extending right across the front face and a rear diaphragm extending right across the rear face whereby the interior space of the loudspeaker is substantially enclosed.
Conveniently the driver unit is located within said space remote from the frame.
Advantageously the driver unit is located outside said space remote from the frame.
Preferably the driver unit is connected to both of said diaphragms whereby the driver unit will apply varying force, corresponding to the varying electric current, to said diaphragms and cause at least one of said diaphragms to flex and emit an acoustic signal from the face of the diaphragm exterior of the loudspeaker.
Conveniently both said diaphragms are flexible and the driver operates the diaphragms in bi-polar mode, that is they move outwards together or inwards together.
Preferably one of said diaphragms is flexible and the other is rigid.
Conveniently the driver unit is mounted to the diaphragm at said one or more selected positions, said positions being remote from the peripheral edge of the diaphragm.
Advantageously the driver unit is mounted on a body which is mounted on pillars to the diaphragm at said positions.
Preferably said positions are selected so that flexure of the diaphragm in one or more of its natural modes of vibration is not impeded.
Said first diaphragm and/or said rear diaphragm may be of double skinned polypropylene copolymer, and may be approximately 3 mm thick.
Conveniently the or each diaphragm is approximately 500 grams or less per square metre, and may have a tensile strength of around 28 MPa or more and may have a Shore hardness of 67 or more.
Preferably the surface of the or each diaphragm is treated with a corona discharge to assist adhesion of paint or paper thereto, or of the diaphragm to the frame.
Conveniently the or each diaphragm has longitudinal ribs forming part of its structure perhaps on one face although in its most preferred form they are internal (ie covered by smooth front ant rear faces). Preferably these ribs are provided by a core of corrugated material or the like and may be square or rectangular and each edge is around 400 mm long.
Advantageously the driver unit includes two co-axial voice coils, each fastened to a different one of said diaphragms, the voice coils co-acting with a magnet suspended centrally between said voice coils.
Conveniently the or each diaphragm is slightly fluted or corrugated or slightly curved, so as to provide a modified acoustic performance of the loudspeaker.
The interior of the loudspeaker may constitutes a sealed box. Alternatively one or more ports or vents may be provided through the frame or through one or both or said diaphragms.
Preferably the first diaphragm is made of a sheet of extruded plastics material having integral front and skins joined by closely spaced parallel walls normal to said skins.
In another aspect the invention provides a loudspeaker including a substantially planar diaphragm which can be vibrated so as to radiate sound from at least the front face thereof, and a driver unit operable by a varying electric current in order to generate a varying force on the diaphragm, the force varying in a manner related to the varying electric current, in which the diaphragm is made of front and rear parallel sheets of material, said sheets being spaced apart by a plurality of walls extending between said sheets and said walls being parallel to each other and extending substantially normal to said sheets, or being of generally rhomboidal or sinusoidal corrugated form.
Preferably the diaphragm is of double skinned polypropylene co-polymer and the walls are substantially normal to said parallel sheets.
Alternatively, the diaphragm is of paper or cardboard, said walls being of substantially corrugated form and being fastened by adhesive to the inner opposing faces of the parallel sheets.
Conveniently the driver unit includes a magnet, one pole of which is in magnetic continuity with a yoke and the other pole of which is positioned from the yoke by an air gap through which gap a voice coil is operable, the voice coil being attached to drive the diaphragm.
Preferable the magnet is a cylindrical permanent magnet and the yoke is co-axial therewith, said air gap being annular. More preferably the magnet is a high strength permanent magnet such as a neodymium magnet.
Advantageously, the yoke is made from low oxygen pure iron annealed very slowly in hydrogen.
Conveniently the voice coil has more windings per unit length thereof away from that axial part of the voice coil which is in said air gap when no electric current is passed through the windings.
For this purpose, the voice coil winding is a single layer, with the windings spaced apart in the region of said axial part. Preferably the voice coil is made of pure copper (although aluminium wire could also be used).
Alternatively or in addition, more than said single layer is wound at regions of the voice coil further from said axial part.
According to another aspect of the present invention there is provided a loudspeaker including a diaphragm which can be vibrated so as to radiate sound from at least the front face thereof, and a driver unit operable by a varying electric current in order to generate a varying force on the diaphragm, the force varying in a manner related to the varying electric current, in which a voice coil of the driver unit is connected to said diaphragm, by means of a panel of material, said panel being of a material and having dimensions such that at a low range of audio frequencies, the voice coil and at least that part of the diaphragm adjacent the panel move substantially at the same amplitude and phase, while at a higher range of audio frequencies at least said part of the diaphragm and a part of the panel adjacent thereto move at a lower amplitude and/or at a different phase from the voice coil, whereby at said higher range a substantial part of the sound emitted by the loudspeaker is radiated from a face of the panel adjacent the voice coil.
Preferably the diaphragm is a planar diaphragm.
Conveniently a sheet of damping material is connected from a stationary part of the loudspeaker to said part of the diaphragm adjacent the panel or to the panel.
The damping material may be cloth or cloth based.
Preferably the damping material is porous or perforated so as to allow the passage of air therethrough.